I am referring ASCE 7-05
-What is the definition of mean height for an arched roof semi circle? will it the total height (i.e radius of circle) or half radius?
- is there a sample of calculation as per asce for wind for cladding of this type of building
-Figure 6-8 page 51 is not clear on defining roof perimeter area for cladding in order to use external pressure coef as per fig 6-11

-What is the definition of mean height for an arched roof semi circle? will it the total height (i.e radius of circle) or half radius?
Look at Note 2 on page 50 and/or Note 1 on Figure 6-16, page 64. Does that answer your question?

- is there a sample of calculation as per asce for wind for cladding of this type of building
Don't know of any myself.

-Figure 6-8 page 51 is not clear on defining roof perimeter area for cladding in order to use external pressure coef as per fig 6-11
I'm not sure I understand your question. Figure 6-8 is for Main Wind Force Resisting System loads and has nothing to do with C&C wind.

Page 50 and Page 64 is for Dome with circular base. in my case it is a normal building with arch roof of semi circle.. th roof is like half cylinder covering the building. does the mean height definition and calculation for the dome apply also for arch roof.

In page 51 , it is stated for arch roof and they have put note 4 for component and cladding where they are referring to go for fig 6-11.

It is confusing that is why I was asking about an example of application.

For a half-cylinder arch I think you would use Figure 6-8 on page 51, and not Figure 6-7 on page 50.

Note 4 would require you to do the following:
1. Compute the spring line angle of your arch. If it is truly a half-circle arch then the spring line angle is 90 degrees.
2. Compute the value of "a" which is the distance from the perimeter inward where the note 4(1) changes to note 4(2). The distance of "a" is shown in the notes of Figure 6-11.
3. Since ASCE 7 is silent on what to do with roof pressures when the slope is > 45 degrees (per the limit in Table 6-11D, one approach would be then to assume your eave, or perimeter edge of roof is the point where the slope of your arch is exactly at 45 degrees slope and tread everything below it as a wall. In doing that you can then take the arch as a smaller arch length and calculate everything accordingly. I'm not sure how valid this is - the commentary doesn't seem to even discuss this so I don't know of a better approach.
4. So you would have wall wind loads below that 45 degree mark and "roof" wind loads, using Figure 6-11D for the perimeter edge roof loads (using the "a" dimension above.

The engineer is wrong on the spring line. The spring line is the point at which the arch "springs" or begins from its base.
At your half-circle, the arch is exactly vertical...thus the angle is 90 degrees from the horizontal per definition of ASCE 7.

Using h as the total height of the circle is also incorrect as ASCE 7 is quite clear that the value of h is the sum of the height at the eave plus the average height of the arch.

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